In recent years evidence has accumulated against immunosurveillance as a primary defense mechanism against neoplasia. Recent reports of tumor specific immunostimulation support the evidence against immunosurveillance. The hypothesis is offered, that immunostimulation of tumors in vertebrates is an evolutionary remnant of a more primitive process seen in invertebrates - immunostimulation of blastemic growth and regeneration. Evidence in support of this hypothesis is presented. The hypothesis not only provides a reason for the existence of cellular immunity but by inference supports the long suggested relationship between the blastema and the neoplasm. The proposed project will test the hypothesis: (1) by both in vivo and in vitro evaluation of the effects of immune stimulation or abrogation of immunity on regenerative ability and growth parameters in earthworms; (2) by determining the susceptibility of earthworms in the non-regenerative state to chemical carcinogenesis; (3) by evaluation of "regeneration field control" in fetal opossum limbs; (4) by evaluation of cellular immune control of regenerative ability in fetal opossums.